The significance of atrial natriuretic factor (ANF) was investigated in the maintenance of the fluid volume in hypovolemia associated with dumping syndrome following gastric resection. The study was performed on 10 patients with Billroth II procedure. Ten age and sex matched patients--without previous gastric surgery served as controls. Each patient underwent oral glucose challenge. The patients following gastric resection underwent an other glucose challenge with intravenous infusion for the maintenance of the fluid volume. All patients with gastric resection showed subjective symptoms of the early dumping syndrome with significant (p less than 0.001) increases in heart rate and in hematocrit, while plasma ANF level decreased significantly (p less than 0.01). Significant negative correlation was found between the changes in hematocrit and the changes in plasma ANF level (r = -0.89; p less than 0.001). Neither the subjective symptoms characteristic for early dumping syndrome, nor changes in the laboratory parameters were noted in the patients during the challenge with infusion. The results show that the hypovolemia in dumping syndrome is associated with significant decreased ANF activity, and in the regulation of ANF release besides the well known stimulating effect of hypervolemia, there exists an inhibition of secretion in volume depleted states.